1. Field of the Invention
The present invention relates to an X-ray computed tomography apparatus, medical image processing apparatus, X-ray computed tomography method, and medical image processing method which scan objects with X-rays.
2. Description of the Related Art
Among X-ray computed tomography apparatuses like that disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 9-19425, a third-generation scanning system is known, which includes an X-ray tube including an X-ray source to produce an X-ray fan beam and a one-dimensional array X-ray detector to acquire projection data from various angles while rotating around an object.
There are available, for example, a conventional scanning system in which an X-ray tube revolves on the same circular orbit and a helical scanning system defined as a scanning system in which an X-ray source and a one-dimensional array X-ray detector continuously rotate around an object, and a bed on which the object is placed moves along the body axis in synchronism with the rotation.
In addition, a conic beam scanning system (also called a multi-slice scanning system) is known, in which an X-ray source to produce an X-ray conic beam in a conical shape is combined with a two-dimensional array detector having a plurality of one-dimensional array detectors stacked in N arrays in the Z-axial direction, and projection data is acquired while the X-ray source and the detector keep facing each other and rotate around an object.
Considering an X-ray beam striking a given detector array, a basic slice thickness in the conic beam scanning system is defined as a thickness in the Z-axial direction when the X-ray beam passes through the rotation center (Z-axis), and an imaging area FOV in the conic beam scanning system is defined as a cylinder having a radius ω centered on the Z-axis.
As a reconstruction processing method to be used when a conic beam scanning system is implemented by a conventional scanning system, the Feldkamp (FDK) reconstruction method is known.
This FDK reconstruction method is an approximate three-dimensional reconstruction algorithm obtained by extending a fan beam (within a two-dimensional plane) reconstruction algorithm, which is a mathematically strict reconstruction method, in the Z-axial direction. This algorithm includes the following steps 1, 2, and 3: (1) multiplying the weighted projection data of projection data by a value dependent on a Z-coordinate; (2) performing convolution computation between the data obtained in step 1 and the same reconstruction function as the fan beam data (convolution computation); and (3) performing back projection of the data obtained in step 2 onto a path through which the X-rays have passed (from the focus to each channel of the detector). Back-projection processing is performed through 360°.
In these reconstruction methods, the central position of each voxel in the imaging area FOV is generally set in correspondence with the central position of a corresponding X-ray detection element of the X-ray detector. That is, the center of each voxel is defined on a line connecting a corresponding X-ray detection element and the X-ray focus. In this case, an X-ray beam has a conic beam shape spreading in the slice direction. Owing to this spread, the degree of interpolation varies depending on the position of each detection element in the slice direction.
The above techniques, however, have the following problem. That is, interpolation becomes uneven depending on the positions of detection elements in the slice direction. For example, almost no interpolation is performed at the central position. This degrades an image SD near the axis of rotation, which is the central position, and the mid-plane, which is a central cross-section in the array direction of the detector. When this is expressed in MIP, a cross-shaped artifact occurs at the central position, as shown in FIG. 12.